Lasers that emit in the visible range have many useful applications. In the medical field, they are used in ophthalmological examinations and surgical procedures. In the communications field, such lasers are useful for transmitters. The blue-green region of the visible spectrum is of special interest because water has a transmission window in that region, and thus lasers that emit in the blue-green region can be used for underwater communications. Such lasers are also useful for welding and cutting operations.
Gas and liquid lasers are known that emit in the blue-green region. Gas and liquid lasers, however, suffer from the disadvantages that they are large, heavy, and very expensive. Another disadvantage is that, frequently, such lasers emit in a narrow spectral region and therefore are not tunable over as wide a range in the sense that that term is used in this specification.
As is well known, lasers require the presence of an optical cavity to provide feedback to allow the emitted light to repeatedly traverse the active medium to establish the conditions for obtaining oscillations. Tuning a laser generally means optimizing the feedback for a particular wavelength while discriminating against all others. But if the lasing medium can only support a narrow frequency band, then the optical cavity must be precisely tuned to that narrow frequency for the system to lase. What is desired is a lasing medium which generates radiation over a relatively broad bandwidth so that tuning of the optical cavity allows any wavelength within that bandwidth to be selected as the output emission. This has not been possible to the best of our knowledge with gas lasers emitting in the visible range.
A solid-state laser has many advantages over a gas or liquid laser, in that it is a more compact construction, lighter in weight, and less fragile. A typical way to pump a solid-state laser is with a known flash lamp which is optically coupled to the active lasing medium. To the best of our knowledge, nobody has ever reported a broad band, tunable solid-state laser that is pumped by a flash lamp and that will emit in the blue-green region of the visible spectrum.
There is much literature that has been published on solid-state lasers. U.S. Pat. No. 4,054,852 describes a solid-state blue-green laser using as the host YLiF.sub.4, LaF.sub.3 and CaF.sub.2, activated with praseodymium.sup.+3 and holmium.sup.+3. However, as reported in this patent, such lasers are typically line emitters and therefore are not tunable.
U.S. Pat. No. 3,624,547 to Dugger describres a germanate system which may be useful in solid-state lasers. The disclosure includes the following compounds as possible hosts: Li.sub.2 Ge.sub.7 O.sub.15, Li.sub.2 GeO.sub.3, Li.sub.2 Ge.sub.4 O.sub.9, Li.sub.4 Ge.sub.9 O.sub.20 and Li.sub.2 Ge.sub.2 O.sub.3. The activators described include chromium, holmium, neodymium, erbium and ytterbium, which are all trivalent. No data is reported in this patent concerning operation of the media described as a laser. In any event, it is not expected that the activators listed would be capable of causing the materials described to emit in the blue-green region of the visible spectrum.
In a concurrently published paper in the Journal of Applied Physics, Vol. 38, No. 5, pages 2345-2349 (April 1967), Dugger lists additional experimental data with respect to some of the materials described in U.S. Pat. No. 3,624,547. No data is reported concerning the use of the material as a solid-state laser. The author mainly concentrates on describing the luminescent properties of some of the germanate compounds activated with the different activators mentioned above.
Finding a proper combination of hosts and activators that can be pumped by a flash lamp to emit broad band radiation in the visible range is an extraordinarily complex task. Some of the problems that are encountered include the following.
First, the material must be capable of being grown as a single crystal containing only a single phase in sufficiently large sizes that a suitable section can be machined which has dimensions adequate for it to function as a lasing medium. The resultant single crystal must be reproducible, preferably transparent, and should luminesce in the desired visible spectrum. There may be a number of materials that would satisfy these conditions, but none so far have been reported as capable of being lased by flash lamp pumping, much less to emit over a broad bandwidth.
A second problem is the necessity that the materials should not suffer damage as a result of the internally generated laser radiation or while undergoing pumping from an external source. This generally requires reasonable thermal conductivity for high power operation.
Third, the resultant band structure of the activated host is crucial to its capability to lase in the visible range. The basic lasing mechanism is for the pumping to excite electrons from some lower state to some higher energy excited state. When sufficient electrons have been created in this excited state so that their population exceeds the population in the lower state, then transitions from the excited state back down to the lower state (directly or via some intermediate level) exceed transitions from the lower state to the excited state, enabling amplification of radiation in the desired spectral range.
However, it has been found that many of these materials have bands or levels at a higher energy than the excited state band. Because of the internal band structure, which incidentally is not at all well understood nowadays, there is a preference for the electrons to undergo competing nonradiative transitions to the higher level rather than radiative transitions down to the lower level, with the net result that no lasing action is possible. The present state of the art does not allow predictions as to what would be the desired band structure to ensure the desired lasing action, much less which hosts activated by which activators would create a theoretically desired band structure.